i imagine you have already checked to see if one of the filaments is burned out...........?
+1That'll 99% likely be the problem. My bike "eats" tail light globes, I probably replace them 3 or 4 times a year (so 7 or 8000km, 4 or 5000miles).
If they weren't so cheap and easy to get and replace, I'd probably investigate some more reliable alternatives (like hunting for some 1152LL "long life" replacement globes, or perhaps an LED replacement taillight), but I know there's 3 or 4 petrol stations on my way home from work who'll sell me an ordinary 1152 globe, and replacing that globe is probably the only useful thing the screwdriver in the underseat toolkit has ever done...
big